Wells are generally drilled into the ground to recover natural deposits of oil and gas, as well as other desirable materials, that are trapped in geological formations in the earth's crust. A well is drilled into the ground and directed to the targeted geological location from a drilling rig at the earth's surface.
Once a formation of interest is reached, drillers often investigate the formation and its contents by taking samples of the formation rock and analyzing the rock samples. Typically, a sample is cored from the formation using a hollow coring bit, and the sample obtained using this method is generally referred to as a “core sample.” Once the core sample has been transported to the surface, it may be analyzed to assess, among other things, the reservoir storage capacity (porosity) and the flow potential (permeability) of the material that makes up the formation; the chemical and mineral composition of the fluids and mineral deposits contained in the pores of the formation; and the irreducible water content of the formation material. The information obtained from analysis of a sample is used to design and implement well completion and production facilities.
“Conventional coring,” or axial coring, involves taking a core sample from the bottom of the well. Typically, this is done after the drill string has been removed, or “tripped,” from the wellbore, and a rotary coring bit with a hollow interior for receiving the core sample is lowered into the well on the end of a drill string. Some drill bits include a coring bit near the center of the drill bit, and a core sample may be taken without having to trip the drill string. A core sample obtained in conventional coring is taken along the path of the wellbore; that is, the core is taken along the axis of the borehole from the rock below the drill bit.
A typical axial core is 4-6 inches (about 10-15 cm) in diameter and can be over 100 feet (about 30 m) long. The rotary motion is typically generated at the surface, and the coring bit is driven into the formation by the weight of the drill string that extends back to the surface. The core sample is broken away from the formation by simply pulling upward on the coring bit that contains the sample.
By contrast, in “sidewall coring,” a core sample is taken from the side wall of a drilled borehole. Sidewall coring is typically performed after the drill string has been removed from the borehole. A wireline coring tool that includes a coring bit is lowered into the borehole, and a small core sample is taken from the sidewall of the borehole. In sidewall coring, the drill string cannot be used to rotate the coring bit, nor can it provide the weight required to drive the bit into the formation. Instead, the coring tool must generate both the rotary motion of the coring bit and the axial force necessary to drive the coring bit into the formation.
In sidewall coring, the available space is limited by the diameter of the borehole. There must be enough space to withdraw and store a sample. Because of this, a typical sidewall core sample is about 1 inch (about 2.5 cm) in diameter and less than about 2 inches long (about 5 cm). The small size of the sample does not permit enough frictional forces between the coring bit and the core sample for the core sample to be removed by simply withdrawing the coring bit. Instead, the coring bit is typically tilted to cause the core sample to fracture and break away from the formation.
An additional problem that may be encountered is that because of the short length of a side wall core sample, it may be difficult to retain the core sample in the coring bit. Thus, a coring bit may also include mechanisms to retain a core sample in the coring bit even after the sample has been fractured or broken from the formation. Sidewall coring is beneficial in wells where the exact depth of the target zone is not well known. Well logging tools, including coring tools, can be lowered into the borehole to evaluate the formations through which the borehole passes. Multiple core samples may be taken at different depths in the borehole so that information may be gained about formations at different depths.
Previous designs, however, are either not suitable for unconsolidated formations or the lower part of the sediment core is disturbed and partly lost during sampling.